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<div class="titlepage"><div><div><h5 class="title">
<a name="spirit.karma.reference.auxiliary.lazy"></a><a class="link" href="lazy.html" title="Lazy Generator (lazy)">Lazy Generator
          (<code class="computeroutput"><span class="identifier">lazy</span></code>)</a>
</h5></div></div></div>
<h6>
<a name="spirit.karma.reference.auxiliary.lazy.h0"></a>
            <span class="phrase"><a name="spirit.karma.reference.auxiliary.lazy.description"></a></span><a class="link" href="lazy.html#spirit.karma.reference.auxiliary.lazy.description">Description</a>
          </h6>
<p>
            The family of <code class="computeroutput"><span class="identifier">lazy</span></code> components
            allows to use a dynamically returned generator component for output generation.
            It calls the provided function or function object at generate time using
            its return value as the actual generator to produce the output.
          </p>
<h6>
<a name="spirit.karma.reference.auxiliary.lazy.h1"></a>
            <span class="phrase"><a name="spirit.karma.reference.auxiliary.lazy.header"></a></span><a class="link" href="lazy.html#spirit.karma.reference.auxiliary.lazy.header">Header</a>
          </h6>
<pre class="programlisting"><span class="comment">// forwards to &lt;boost/spirit/home/karma/auxiliary/lazy.hpp&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">spirit</span><span class="special">/</span><span class="identifier">include</span><span class="special">/</span><span class="identifier">karma_lazy</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
</pre>
<p>
            Also, see <a class="link" href="../../../structure/include.html" title="Include">Include Structure</a>.
          </p>
<h6>
<a name="spirit.karma.reference.auxiliary.lazy.h2"></a>
            <span class="phrase"><a name="spirit.karma.reference.auxiliary.lazy.namespace"></a></span><a class="link" href="lazy.html#spirit.karma.reference.auxiliary.lazy.namespace">Namespace</a>
          </h6>
<div class="informaltable"><table class="table">
<colgroup><col></colgroup>
<thead><tr><th>
                    <p>
                      Name
                    </p>
                  </th></tr></thead>
<tbody><tr><td>
                    <p>
                      <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">lazy</span> <span class="comment">// alias:
                      boost::spirit::karma::lazy</span></code>
                    </p>
                  </td></tr></tbody>
</table></div>
<h6>
<a name="spirit.karma.reference.auxiliary.lazy.h3"></a>
            <span class="phrase"><a name="spirit.karma.reference.auxiliary.lazy.model_of"></a></span><a class="link" href="lazy.html#spirit.karma.reference.auxiliary.lazy.model_of">Model
            of</a>
          </h6>
<div class="blockquote"><blockquote class="blockquote"><p>
              <a class="link" href="../generator_concepts/generator.html" title="Generator"><code class="computeroutput"><span class="identifier">Generator</span></code></a>
            </p></blockquote></div>
<div class="variablelist">
<p class="title"><b>Notation</b></p>
<dl class="variablelist">
<dt><span class="term"><code class="computeroutput"><span class="identifier">fg</span></code></span></dt>
<dd><p>
                  A function or function object that evaluates to a generator object
                  (an object exposing the <a class="link" href="../generator_concepts/generator.html" title="Generator"><code class="computeroutput"><span class="identifier">Generator</span></code></a>). This function
                  will be invoked at generate time.
                </p></dd>
</dl>
</div>
<p>
            The signature of <code class="computeroutput"><span class="identifier">fg</span></code> is
            expected to be
          </p>
<pre class="programlisting"><span class="identifier">G</span> <span class="identifier">f</span><span class="special">(</span><span class="identifier">Unused</span><span class="special">,</span> <span class="identifier">Context</span><span class="special">)</span>
</pre>
<p>
            where <code class="computeroutput"><span class="identifier">G</span></code>, the function's
            return value, is the type of the generator to be invoked, and <code class="computeroutput"><span class="identifier">Context</span></code> is the generator's Context
            type (The first argument is <code class="computeroutput"><span class="identifier">unused</span></code>
            to make the <code class="computeroutput"><span class="identifier">Context</span></code> the
            second argument. This is done for uniformity with <a class="link" href="../../tutorials/semantic_actions.html" title="Generator Semantic Actions">Semantic
            Actions</a>).
          </p>
<h6>
<a name="spirit.karma.reference.auxiliary.lazy.h4"></a>
            <span class="phrase"><a name="spirit.karma.reference.auxiliary.lazy.expression_semantics"></a></span><a class="link" href="lazy.html#spirit.karma.reference.auxiliary.lazy.expression_semantics">Expression
            Semantics</a>
          </h6>
<p>
            Semantics of an expression is defined only where it differs from, or
            is not defined in <a class="link" href="../generator_concepts/generator.html" title="Generator"><code class="computeroutput"><span class="identifier">Generator</span></code></a>.
          </p>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
                    <p>
                      Expression
                    </p>
                  </th>
<th>
                    <p>
                      Semantics
                    </p>
                  </th>
</tr></thead>
<tbody>
<tr>
<td>
                    <p>
                      <code class="computeroutput"><span class="identifier">fg</span></code>
                    </p>
                  </td>
<td>
                    <p>
                      The <a href="../../../../../../../../libs/phoenix/doc/html/index.html" target="_top">Boost.Phoenix</a>
                      function object <code class="computeroutput"><span class="identifier">fg</span></code>
                      will be invoked at generate time. It is expected to return
                      a generator instance. This generator is then invoked in order
                      to generate the output. This generator will succeed as long
                      as the invoked generated succeeds as well (unless the underlying
                      output stream reports an error).
                    </p>
                  </td>
</tr>
<tr>
<td>
                    <p>
                      <code class="computeroutput"><span class="identifier">lazy</span><span class="special">(</span><span class="identifier">fg</span><span class="special">)</span></code>
                    </p>
                  </td>
<td>
                    <p>
                      The function or function object will be invoked at generate
                      time. It is expected to return a generator instance (note this
                      version of <code class="computeroutput"><span class="identifier">lazy</span></code>
                      does not require <code class="computeroutput"><span class="identifier">fg</span></code>
                      to be a <a href="../../../../../../../../libs/phoenix/doc/html/index.html" target="_top">Boost.Phoenix</a>
                      function object). This generator is then invoked in order to
                      generate the output. This generator will succeed as long as
                      the invoked generated succeeds as well (except if the underlying
                      output stream reports an error).
                    </p>
                  </td>
</tr>
</tbody>
</table></div>
<h6>
<a name="spirit.karma.reference.auxiliary.lazy.h5"></a>
            <span class="phrase"><a name="spirit.karma.reference.auxiliary.lazy.attributes"></a></span><a class="link" href="lazy.html#spirit.karma.reference.auxiliary.lazy.attributes">Attributes</a>
          </h6>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
                    <p>
                      Expression
                    </p>
                  </th>
<th>
                    <p>
                      Attribute
                    </p>
                  </th>
</tr></thead>
<tbody>
<tr>
<td>
                    <p>
                      <code class="computeroutput"><span class="identifier">fg</span></code>
                    </p>
                  </td>
<td>
                    <p>
                      The attribute type <code class="computeroutput"><span class="identifier">G</span></code>
                      as exposed by the generator <code class="computeroutput"><span class="identifier">g</span></code>
                      returned from <code class="computeroutput"><span class="identifier">fg</span></code>.
                    </p>
                  </td>
</tr>
<tr>
<td>
                    <p>
                      <code class="computeroutput"><span class="identifier">lazy</span><span class="special">(</span><span class="identifier">fg</span><span class="special">)</span></code>
                    </p>
                  </td>
<td>
                    <p>
                      The attribute type <code class="computeroutput"><span class="identifier">G</span></code>
                      as exposed by the generator <code class="computeroutput"><span class="identifier">g</span></code>
                      returned from <code class="computeroutput"><span class="identifier">fg</span></code>.
                    </p>
                  </td>
</tr>
</tbody>
</table></div>
<h6>
<a name="spirit.karma.reference.auxiliary.lazy.h6"></a>
            <span class="phrase"><a name="spirit.karma.reference.auxiliary.lazy.complexity"></a></span><a class="link" href="lazy.html#spirit.karma.reference.auxiliary.lazy.complexity">Complexity</a>
          </h6>
<p>
            The complexity of the <code class="computeroutput"><span class="identifier">lazy</span></code>
            component is determined by the complexity of the generator returned from
            <code class="computeroutput"><span class="identifier">fg</span></code>.
          </p>
<h6>
<a name="spirit.karma.reference.auxiliary.lazy.h7"></a>
            <span class="phrase"><a name="spirit.karma.reference.auxiliary.lazy.example"></a></span><a class="link" href="lazy.html#spirit.karma.reference.auxiliary.lazy.example">Example</a>
          </h6>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
              The test harness for the example(s) below is presented in the <a class="link" href="../basics.html#spirit.karma.reference.basics.examples">Basics Examples</a>
              section.
            </p></td></tr>
</table></div>
<p>
            Some includes:
          </p>
<p>
</p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">spirit</span><span class="special">/</span><span class="identifier">include</span><span class="special">/</span><span class="identifier">karma</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">spirit</span><span class="special">/</span><span class="identifier">include</span><span class="special">/</span><span class="identifier">support_utree</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">phoenix</span><span class="special">/</span><span class="identifier">core</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">phoenix</span><span class="special">/</span><span class="keyword">operator</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">fusion</span><span class="special">/</span><span class="identifier">include</span><span class="special">/</span><span class="identifier">std_pair</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">deep_copy</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">iostream</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">string</span><span class="special">&gt;</span>
</pre>
<p>
          </p>
<p>
            Some using declarations:
          </p>
<p>
</p>
<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">karma</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">karma</span><span class="special">;</span>
<span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">karma</span><span class="special">::</span><span class="identifier">_1</span><span class="special">;</span>
<span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">ascii</span><span class="special">::</span><span class="identifier">string</span><span class="special">;</span>
<span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">phoenix</span><span class="special">::</span><span class="identifier">val</span><span class="special">;</span>
</pre>
<p>
          </p>
<p>
            Basic usage of the <code class="computeroutput"><span class="identifier">lazy</span></code>
            generator:
          </p>
<p>
</p>
<pre class="programlisting"><span class="identifier">test_generator_attr</span><span class="special">(</span><span class="string">"abc"</span><span class="special">,</span> <span class="identifier">karma</span><span class="special">::</span><span class="identifier">lazy</span><span class="special">(</span><span class="identifier">val</span><span class="special">(</span><span class="identifier">string</span><span class="special">)),</span> <span class="string">"abc"</span><span class="special">);</span>
<span class="identifier">test_generator</span><span class="special">(</span><span class="string">"abc"</span><span class="special">,</span> <span class="identifier">karma</span><span class="special">::</span><span class="identifier">lazy</span><span class="special">(</span><span class="identifier">val</span><span class="special">(</span><span class="identifier">string</span><span class="special">))[</span><span class="identifier">_1</span> <span class="special">=</span> <span class="string">"abc"</span><span class="special">]);</span>
</pre>
<p>
          </p>
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<td align="right"><div class="copyright-footer">Copyright © 2001-2011 Joel de Guzman, Hartmut Kaiser<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
      </p>
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